When used to charge an imaging member, a roller used to create a charge on another surface or substrate is commonly referred to as bias charge roll (“BCR”). When used to charge a substrate to enable transfer of a developed image from an imaging member to a substrate member, a roller used to create such bias charging is commonly referred to as a bias transfer roll (“BTR”). Although both may differ in details particular to their applications, both represent illustrative embodiments of the present invention.
Generally, the process of electrostatographic reproduction is initiated by substantially uniformly charging a photoreceptive member, followed by exposing a light image of an original document thereon. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface layer in areas corresponding to non-image areas in the original document, while maintaining the charge on image areas for creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which a charged developing material is deposited onto the photoconductive surface layer, such that the developing material is attracted to the charged image areas on the photoreceptive member. Thereafter, the developing material is transferred from the photoreceptive member to a copy sheet or some other image support substrate to which the image may be permanently affixed for producing a reproduction of the original document. In a final step in the process, the photoconductive surface layer of the photoreceptive member is cleaned to remove any residual developing material therefrom, in preparation for successive imaging cycles.
The above described electrostatographic reproduction process is well known and is useful for both digital copying and printing as well as for light lens copying from an original. In many of these applications, the process described above operates to form a latent image on an imaging member by discharge of the charge in locations in which light from a lens, laser, or LED discharges a charge. Such printing processes typically develop toner on the discharged area, known as DAD, or “write black” systems.
As an alternative to corona generating devices used in charging systems, roll charging systems such as, BCR's and BTR's have been developed and incorporated into various machine environments with limited success. BCR charging systems are exemplified by U.S. Pat. No. 2,912,586, to R. W. Gundlach; U.S. Pat. No. 3,043,684, to E. F. Mayer; U.S. Pat. No. 3,398,336, to R. W. Martel et al.; U.S. Pat. No. 3,684,364, to F. W. Schmidlin; and U.S. Pat. No. 3,702,482, to Dolcimascolo et al., among others, wherein an electrically biased charging roller is placed in contact with the surface to be charged, e.g. the photoreceptive member. Also relevant is U.S. Pat. No. 5,412,455, to Ono et al. wherein a charging device includes: a member to be charged; a charging member connectable to the member to be charged; a power source for supplying an oscillating voltage to the charging member; and a constant voltage element connected electrically in parallel with the power source for generating the oscillating voltage. Also, U.S. Pat. No. 5,463,450, to Inoue et al. discloses a charging apparatus for electrically charging a member to be charged including a charging member contactable to the member to be charged. The member to be charged includes a core and a voltage source for applying an oscillating voltage between the member to be charged and the charging member, wherein the frequency of the oscillating voltage satisfies a predetermined condition. Each of these is hereby incorporated by reference in their entirety.
Additionally, BCR charging systems may be operated in a DC voltage only mode. While this offers less wear and chemical interaction with the photoreceptor surface, the charge uniformity is much more sensitive to toner and additive contamination on the surface of the bias charging roll. This mode of charging typically requires a robust method of keeping the BCR surface clean to achieve good photoreceptor charge uniformity.
In BTR charging systems, DC voltage is typically used. DC voltage attracts dirt, however, especially toner in spaces void of printing substrates, such spaces comprising inter-document zones, areas exposed when printing on less-than-full-width printing media, and similar areas in which the BTR is directly exposed to the charge carrying member or intermediate transfer member.
The top failure mode seen in xerographic systems that use bias charging roll (BCR) architecture is non-uniform halftones due to contamination on the BCR surface. During operation, toner additives that are not cleaned off of the photoreceptor become attached to the surface of the BCR and form localized bands around the roll. These bands then create non-uniform voltage during the charging step, which lead to halftone streaks in the process direction. Current commerically available charge device cleaners are plagued with contaminate capacity problems. In the cross process direction of device rotation, the same area of the roll cleaner comes into the same area of the roll. Over many revolutions, the cleaner fills up with contamination and stops cleaning the surface of the roll. These uncleaned areas create non-uniform charge of the receptor and show up a streaks in halftone areas of the output.
The following invention provides a method to clean a BCR in order to extend xerographic module life and providing uniform halftones. Toner additive and other contamination is removed from the roll by fabric held in contact with the roll. The fabric is continuously replenished by using a web architecture similar to that used in fusing systems. The amount of fabric can be selected based on how long the charging system needs to be used. Fresh fabric is brought into contact with the roll at periodic intervals in order to provide a new cleaning surface against the roll.
In accordance with one embodiment of the present invention, there is provided a xerographic printing machine having an apparatus for applying an electrical charge to a member to be charged, comprising: a bias roll member situated in contact with a surface of the member to be charged; means for applying an electrical bias to said bias roll member; and a cleaning mechanism for cleaning said bias roll member, said cleaning mechanism comprises an elongated web of cleaning cloth material being translated in respect to said bias roll member, and wherein said bias roll member is rotated in the process direction and said elongated web of cleaning cloth material is translated in a direction being substantially transverse to the process direction.